When you start system, does not grub menu appear with the entries that os-prober shows it found as shown in your post #22?
And you scroll down to which ever grub entry you want to boot.

Several examples, but has fancy background images & colors.
https://help.ubuntu.com/community/Ma...tomGrub2Screen

More examples, started on last page of a mega-thread.
https://ubuntuforums.org/showthread....076205&page=58